(1) Field of the Invention
This invention relates to an improvement in an oxygen sensor employing a plate form oxygen sensor element for detecting the concentration of oxygen in a gas under measurement, particularly an exhaust gas from an internal-combustion engine.
(2) Description of the Prior Art
As a so-called oxygen sensor, there has been known a system for detecting the concentration of oxygen in an exhaust gas from an internal-combustion engine and optimizing the combustion condition of the engine based on the detection signal, in order to achieve clarification of the exhaust gas, savings of fuel cost, etc.
In recent years, elongate plate form oxygen sensor elements have been proposed for use in the aforementioned oxygen sensor, from such viewpoints as ease of manufacture and ease of obtaining a compact design.
This type of oxygen sensor elements are constructed, for example, as shown in FIG. 6, in which a solid electrolyte capable of oxygen-ionic conduction and comprising zirconia as a main constituent is formed into an elongate plate-like shape, a measuring electrode 32 to be exposed to a gas under measurement is provided on a surface of an end portion of the thus formed solid electrolyte plates 31 (31a, 31b, 31c and 31d), whereas an inlet passage (in FIG. 6, represented by a cavity 43) for a reference gas (generally, the atmospheric air is used) opened at one end is formed in an oxygen sensor 51 along the longitudinal direction, and a reference electrode 34 is disposed at a depth portion of the reference gas inlet passage so as to face the measuring electrode 32. In addition to the electrodes 32 and 34, the oxygen sensor 10 may have an oxygen pumping electrode (not shown) or may have a heater 38 (FIG. 6) or the like, according to the intended use thereof.
For connection of the electrodes 32, 34 and the heater 38 to an external circuit, conductor leads (a conductor lead 33 for the measuring electrode, a conductor lead 35 for the reference electrode, and a conductor lead 39 for the heater) are provided extending from the electrodes 32, 34 and the heater 38 in the longitudinal direction of the oxygen sensor element 51. The conductor leads are so arranged as to reach electrode terminal portions 61 (61a, 61b, 61c and 61d) located near an end portion of the oxygen sensor element 51 on the side opposite to the side on which the aforementioned electrodes and the like are provided.
In order that lead wires 20 (FIG. 5) used for connection to an external circuit may be connected advantageously (with or without use of connectors), it is desirable that end portions of the conductor leads, on the side opposite to the side of the electrodes 32, 34 and the heater 38, be all disposed on the surface of the oxygen sensor element 51.
Therefore, the conductor leads making conduction to members disposed inside the oxygen sensor element 51, such as the reference electrode 34 and the heater 38, should be led out from the inside to the surface of the oxygen sensor element 51. This is accomplished, for example, by a method in which the conductor leads are led out to the surface of the oxygen sensor element 51 via through-holes 36 (36a, 36b and 36c) formed in the thickness direction of the oxygen sensor element 51.
Besides, in order to avoid needless lengthening of the conductor leads or to dispose the electrode terminal portions 61 as closer as possible to the rear end side (A-side) of the oxygen sensor element 51, the through-holes 36 have conventionally been located nearer to the front end (or tip) of the oxygen sensor element 51 than are the electrode terminal portions 61.
When the through-holes 36 are thus provided in the oxygen sensor element 51 formed mainly of a ceramic, however, the mechanical strength of the oxygen sensor element 51 at the portion of the through-holes 36 is lowered due to a reduction in the sectional area of the elongate plate form sensor element 51, a concentration of stress, etc.
This will be explained more in detail below.
FIG. 5 shows the construction of an oxygen sensor employing the oxygen sensor element 51 shown in FIG. 6. In FIG. 5, the oxygen sensor element 51 is held stationary by a metallic housing 11 and a cylindrical metallic inner tube 12 welded thereto, through the action of talc 15 (15a and 15b) packed in the spaces between ceramic supporters 14 (14a, 14b and 14c). For protection of the oxygen sensor element 51 from external environments, a metallic outer tube 13 is fitted to the outer peripheral portion of an upper annular projection of the housing 11 and fixed in position by welding.
In the construction of the oxygen sensor, the contacts 16 are pressed against the electrode terminal portions 61 with a pressure not lower than a predetermined value, at contact point portions between the electrode terminal portions 61 and the contacts 16. Therefore, there is only a low degree of freedom between the oxygen sensor element 51 and a contact member 26. Thus there has been the problem that when an excessive bending moment is exerted on the portion of the oxygen sensor element 51 above the top surface of the talc 15b in assembling the oxygen sensor or in other situations, a bending moment is exerted also on the portion of the through-holes 36, which is relatively lower in mechanical strength, resulting in breakage of the sensor element at that portion.
There has also been the problem that an excessive external shock exerted on the oxygen sensor 10 may vibrate the contact member 26, the mass of which is somewhat large, whereby a bending moment is exerted on the oxygen sensor element 51, resulting in breakage of the sensor element at the portion of the through-holes 36, similarly to the above case.
Referring now to FIG. 3, there are shown a schematic view and a moment diagram representing the condition where a moment and a load are simultaneously exerted on the contact point portion. As is clear from FIG. 3, a bending moment acts also on the through-hole position, where the mechanical strength is relatively lower, so that the element may be broken at the portion of the through-holes 36.